In the central nervous system (CNS), measles virus produces not only acute, post infective encephalomyelitis but also Subacute Sclerosing Panencephalitis (SSPE), a progressive inflammatory disease of children after viral latency of several years. Measles has been implicated in the etiology of Multiple Sclerosis (MS) as well. In these diseases the CNS virus appears to be defective or incomplete and the pathogenesis of them may involve immunopathological responses to the defective, latent infection. A latent CNS infection of immature, or transiently immunosuppressed adult hamsters with hamster adapted SSPE agent has been developed which has many of the virologic, serologic and ultrastructural characteristics of human SSPE. The CNS virus changes from a complete to a defective, cell-associated form coincident with the appearance of host serum antibodies to measles. Using virological, serological, histological, immunofluorescent, electron microscopic and ultrastructural immunoperoxidase methods we propose sequential studies to define more completely characteristics of the latent CNS virus and the host immune response to it. An investigation combining the latent infection with experimental allergic encephalomyelitis has been initiated and will be extended. Study of this chronic, cell-associated CNS viral infection with measles virus will increase our knowledge of SSPE and the findings may also be significant in understanding Multiple Sclerosis. It may also aid our understanding of cellular immune mechanisms as they operate in the CNS and advance the evaluation of new forms of therapy for SSPE and Multiple Sclerosis.